The present invention relates to the dispensing of chemicals, and has its most important application to the dispensing of developing and finishing chemicals upon the ultraviolet light exposed surfaces of lithographic-offset printing plates. Such printing plates are commonly formed by a process involving the exposure of selected areas of a photosensitive material coated on the plates. Images are developed on each plate by applying a developing liquid thereto which removes the unexposed portions of the photosensitive coating of the plate. To minimize the cost of making such printing plates, the application of developing and finishing chemicals to these plates has been heretofore carried out on a semi-automated basis. Thus, printing plate developing equipment is generally utilized wherein each printing plate, after exposure to ultraviolet light through a negative, is presented to the inlet station of the equipment which includes a conveyer for moving the exposed plate continuously past a developing station where a developing chemical is desirably evenly applied thereto, a washing station where the developing chemical is washed from the surface of the plate, and finally to a finishing station where a finishing chemical is desirably evenly applied thereto.
This equipment has left much to be desired from the standpoint of its reliability of operation, and ease of maintenance and set-up for operation. For example, the printing and finishing chemicals have been heretofore applied to the exposed printing plates by controlling the opening and closing of a valve to which the developing or finishing chemical is fed under pressure by a pump. Supply conduits extend from the control valve and one or more dispensing tubes remotely located from the valve. A number of such dispensing tubes are commonly laterally spaced across the chemical dispensing station involved for dropping small amounts of the chemical involved at different points across the width of the printing plate moving longitudinally past the same. Spreader brushes are used to spread the chemical evenly across the plate. It is especially important that the amount of developing chemical which is dispensed across the width of each plate be uniform, and so it is important that each dispensing tube discharges about the same amount of developing chemical and with the same viscosity or concentration. The valves which control the flow of the chemical to the various remote dispensing tubes normally operate automatically as the printing plate is fed to the input station of the equipment, where a micro-switch is located for opening the valve until the plate passes thereby.
The developing and finishing chemicals generally comprise a carrier liquid with distributed particles of the developing or finishing chemical therein which can settle out therefrom. Therefore, if an appreciable amount of time lapses between the feeding of successive printing plates to the inlet station of the equipment, the developing or finishing chemical remaining in various parts of the conduit system leading to and extending from the control valve will become non-homogeneous, as the particles in the carrier liquid settle out, sometimes even clogging the valve or conduit to a point where less than the desired flow rate of the chemical is obtained. In such case, when the next printing plate is applied to the equipment, at least the initial onrush of the chemical will be inadequate or non-homegenous and of an undesired concentration. Of course, the equipment could be designed so that the initial onrush of the chemical being dispensed is dropped ahead of the printing plate if the system is not completely clogged, but this complicates the design of the equipment and is wasteful of the chemical and/or slows down the speed of the equipment. Also, the use of a pump to feed the chemical through a conduit system over different conduit lengths can result in unequal discharge pressures at each dispensing tube, resulting in different amounts of chemical dispensed across the width of each printing plate.
It is, accordingly, one of the objects of the present invention to provide a chemical dispensing system, most advantageously one having utility in dispensing developing and finishing chemicals to a printing plate developing system as described, which minimizes or avoids problems referred to, namely the varying or uneven concentration or value of liquid chemicals applied across the printing plate or other article involved.
A related object of the invention is to provide chemical dispensing equipment as described which has easy to operate set-up and flow rate adjustment controls so that the equipment can be set-up and used reliably with chemicals requiring different flow rates. A still further related object of the invention is to provide chemical dispensing equipment which makes it easy initially to flush out the equipment, to initially get rid of any undesirably thick or viscous settled out chemical which could otherwise clog up the equipment.